The studies outlined in this proposal address the hypothesis that a synaptic interaction exists between immune and neural cells within specialized immunosensory tissues. This proposal is based upon anatomical observations that reveal close physical relationships between immune cells and neurons within a circumventricular organ of the brainstem, the area postrema, and of neurons and chemoreceptive cells associated with the vagus nerve and paraganglia. These relationships are so intimate that they can't be resolved at the light microscopic level. The purpose of the proposed studies is to apply more powerful electron microscopic analysis to the issue of the precise nature of the physical relationships between immune cells and neurons in these two immunosensory structures and the chemical mediators with which they communicate. Thus our specific aims are to identify appositions and/or synapses between immune and neural cells in specialized immunosensory tissues at the ultrastructural level. These studies will use double immunolabeling for electron microscopy to identify immune cells (dendritic cells and mast cells) and their neuronal targets within the area postrema, vagus nerve and paraganglia. In addition, the expression of many immune-derived mediators must be induced by immune activation. Thus, a second aim is to identify potential inducible immune-derived mediators and their relationships to neural and/or chemoreceptive elements. These studies will utilize multiple label immunofluorescence and double label immuno-electron microscopy in both the area postrema and vagus nerve and paraganglia from animals treated with immune stimulants, to provide clues to type of chemical communication pathways (synaptic or paracrine) these immune cells utilize to modulate neuronal function. We believe the studies described in this proposal are appropriate for the exploratory R21 funding mechanism because, although the methodology proposed is not new, these studies apply a novel approach, ultrastructural analysis, to the investigation of immune-neural interactions. Further, a synaptic communication pathway between immune cells and neurons differs dramatically from the well-established endocrine mechanisms. This kind of interaction could provide the means by which immune cells can target specific neuronal populations, fine-tuning the brain's response to infection. In addition, the dynamic changes evident in immune cells concomitant with infection may induce synaptogenesis, and thus provide a model for the study of activity-dependent synaptic plasticity. Finally, the interactions between immune and neural cells may well be bi-directional, providing a novel mechanism by which neural activity can modulate immune cell functions.